1. Field of the Invention
The present invention generally relates to an image forming apparatus, and in particular to a developer density detecting device for detecting density of a liquid type developer having high viscosity which is utilized in a developing device for developing a latent image formed on an image carrier.
2. Description of the Background
In a background image forming apparatus having a developing device for developing a latent image formed on an image carrier with a liquid type developer, a density of the liquid type developer is detected generally by an optical sensing device including a light emitting element for emitting a light beam and a light sensing element. Die light sensing element receives the light beam emitted from the light emitting element after passing through a liquid type developer existing between the light emitting element and the light sensing element.
In such an optical sensing device, the light sensing element generates a voltage signal indicative of an intensity of the light beam received by the light sensing element, which corresponds to a density of the liquid type developer existing between the light emitting element and the light sensing element which receives a light beam emitted from the light emitting element after passing through the liquid type developer. Thereby, the weaker the light beam received by the light sensing element, the higher the density of the liquid type developer.
Further, a background liquid type developer is always stirred by a fan or the like disposed in a developing device so as to eliminate unevenness thereof during an operation of an image forming apparatus. However, even if such a liquid type developer is always circulated by a fan or the like as described above, a density of such a liquid type developer may not be precisely detected by the background optical sensing device when an image forming apparatus operates for a long time, since a quantity of liquid type developer having a high viscosity adheres to the light sensing element for sensing the light beam over time, or a quantity of the liquid type developer drifts at a position between the light emitting element and the light sensing element over time. As a result, an optical beam emitted by the light emitting element is excessively prevented from arriving at the light sensing element by the liquid type developer adhering to the light sensing element and/or drifting between the light emitting element and the light sensing element. As a result, an actual density of a liquid type developer is not accurately detected.
Further, density detection of a liquid type developer by the background optical sensing device is not entirely stable during a long operation of an image forming apparatus due to variations in a level of adhering of the liquid type developer to a light sensing element or a level of drifting thereof at a position between a light emitting element and a light sensing element.
Further, since, in recent years, a darker liquid type developer has been used in an image forming apparatus, in a copier or a printer, for example, having a large CPM (copy per minute), occasions of the liquid type developer adhering to a light sensing element and/or drifting thereof at a position between the light emitting element and the light sensing element is increasing.
Further, a light sensing element of such an optical sensing device generates a different voltage when a different color liquid type developer, a blue liquid type developer for example, is used since a difference exists in the amount of light beam passage through liquid type developer of different colors. Accordingly, if a color of a liquid type developer is changed, a voltage generated by a light sensing element needs to be adjusted to enable using a common control device as previously used for a different color liquid type developer. Therefore, an optical sensing device is generally not suitable for detecting a liquid type developer when an image forming apparatus changes a color of a liquid type developer.